Brown the hamburger and then add the taco seasoning, according to package directions.
Preheat the oven to 350. In a large bowl, combine the corn bread, egg and milk until blended. Spread into an 8 x 8 pan sprayed with cooking spray. Bake for 15 minutes.
Combine taco meat and beans, spread over corn bread. Combine sour cream, 3/4 cup cheese and onion. Spread over meat mixture. Bake for 20 - 25 minutes longer or until heated through and cheese is melted. Sprinkle with tomato, lettuce and remaining 1/4 cup of cheese.